A known spark plug for providing ignition in an internal combustion engine, such as an automobile engine, has, for example, a structure shown in FIG. 10 (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-129398). This spark plug 201 includes an insulation member (ceramic insulator) 1 made of ceramic, assuming the form of a hollow, tubular shaft, and having a center electrode 5 projecting from the front end (lower end in FIG. 10) thereof, and a tubular metallic shell 21, which fixedly holds the insulation member 1 in a surrounding manner. The metallic shell 21 is formed such that the inner circumferential surface is greater in diameter (inside diameter) at a rear portion than at a front portion. The metallic shell 21 has an annular support ledge 24 on the inner circumferential surface of its portion located toward its front end. The support ledge 24 assumes the form of a surface facing rearward and supports an annular butt portion 4, which assumes the form of a surface facing frontward and is provided on the outer circumferential surface of the insulation member 1. In the present patent application, the “front end” refers to the lower end, in FIG. 10, of the spark plug or its component members and regions (or portions), such as the metallic shell 21 and the insulation member 1, and the “rear end” refers to an opposite end (upper end) of the front end.
Meanwhile, the insulation member 1 has the annular butt portion 4, which assumes the form of a surface facing frontward. Butt portion 4 is located at the rear end of a front end shaft portion 7, which is located toward the front end of the insulation member 1 and is tapered frontward. The insulation member 1 is disposed internally of the metallic shell 21 such that the butt portion 4 butts against the support ledge 24 mentioned above. An annular (tubular) insulation space K is formed between the front end shaft portion 7 of the insulation member 1 and the inner circumferential surface of the metallic shell 21. The insulation member 1 has a mating shaft portion 10 located rearward of the front end shaft portion 7 and greater in diameter than the front end shaft portion 7. The mating shaft portion is disposed in a mating hole portion 30 of the metallic shell 21 in a loose fit condition.
In the spark plug (hereinafter, may be referred to merely as plug) 201, the insulation member 1 in which the center electrode 5, etc., are fixed, is inserted into the metallic shell 21 from the rear end of the metallic shell 21 and disposed such that the butt portion 4 butts against the support ledge 24 of the inner circumferential surface of the metallic shell 21 via a ring-like flat packing (metal packing) 42. Then, a crimp portion 39 at the rear end of the metallic shell 21 is bent toward an axis G (radially inward) in such a manner as to cover a facing-rearward surface 14 of a flange-like large-diameter shaft portion 12 located in an axially intermediate region of the insulation member 1, and is strongly pressed frontward, thereby fixing the insulation member 1 in the metallic shell 21. That is, the support ledge 24 of the metallic shell 21 and the butt portion 4 of the insulation member 1 are pressed against each other with the flat packing 42 intervening therebetween, thereby maintaining gas tightness therebetween and fixing the insulation member 1 in a condition in which the insulation member 1 is pressed against a front end portion of the metallic shell 21.
The thus-configured plug 201 is mounted, for use, into a plug hole (threaded hole) of an unillustrated engine head via a mounting screw 25 provided on the outer circumference thereof. At this time, since gas tightness is maintained between the support ledge 24, the flat packing 42, and the butt portion 4, outward leakage of fuel gas (hereinafter, referred to merely as gas) from a cylinder is prevented. Also, heat of the center electrode 5 and the insulation member 1, which assume high temperature as a result of ignition of gas, is propagated (transmitted) to the engine head via the flat packing 42 and the metallic shell 21, thereby preventing an increase in temperature of the front end of the insulation member 1, etc.
During operation of an engine, a front end region of the spark plug 201 assumes high temperature and is exposed to gas blast at all times. Meanwhile, the metallic shell 21 is usually made of an iron-based metal, whereas the insulation member 1 is made of ceramic. Accordingly, when the front end region of the spark plug 201 assumes high temperature, the amount of thermal expansion of the metallic shell 21 becomes far greater than that of the insulation member 1, since the thermal expansion coefficient of the metallic shell 21 is far greater than that of the insulation member 1. Even though such a thermal expansion difference exists between the two members, the thermal expansion difference does not immediately affect the spark plug 201, since the spark plug 201 is assembled in a condition in which the crimp portion 39 at the rear end of the metallic shell 21 is compressively deformed frontward to thereby strongly press the insulation member 1 frontward.
However, since the expansion of the metallic shell 21 along the direction of the axis G is far greater than that of the insulation member 1, the force with which the butt portion 4 of the insulation member 1 is pressed against the support ledge 24 of the metallic shell 21 unavoidably decreases with age. Continuation of such a condition leads to impairment in gas tightness. Eventually, as shown in FIG. 11, a clearance (small clearance) is formed between the support ledge 24, the flat packing 42, and the butt portion 4, resulting in outward discharge of gas through the clearance. Such a problem is likely to worsen as the length of the metallic shell 21 along the axis G and the length of the mounting screw 25 increase.
Also, impairment in gas tightness and the formation of the clearance results in impairment in heat transference for transferring heat of a front end portion of the insulation member 1 to the engine head. As mentioned above, heat of the front end portion of the insulation member 1 is transferred to the engine head via a gas tightness-maintaining portion consisting of the butt portion 4, the flat packing 42, and the metallic shell 21. However, the formation of a clearance in the gas tightness-maintaining portion causes impairment in heat transference. As a result, a front end region of the insulation member 1 and the center electrode 5 assume excessively high temperature, potentially resulting in occurrence of preignition or thermal erosion of the electrode.
The present invention has been conceived in view of the above problem in a spark plug having the above-mentioned structure or configuration, and an object of the present invention is to prevent impairment in gas tightness between a metallic shell and an insulation member in a plug configured such that the insulation member is pressed frontward in the metallic shell to thereby be fixedly held in place.